Modulation system



Dec. 10, 1940. DfPoLLAcK MODULATION SYSTEM Filed Sept. 30, 1938 Ism-nemer CIttorn-eg 35 tem therein described. The general system bodiment of my invention- Patented Dec'. 10, 1940 I A l UNITED STATES,I PATENT OFFICE MODULTION SYSTEM Dale Pollack, Haddon Heights, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application September 30, 1938, Serial No. 232,483 Claims. (Cl. 179-1715) My invention relates to modulation systems phase auxiliary currents, a pair of auxiliary powand more particularly to a device for modulating er tubes are directly coupled to the antenna, and radio frequency currents in which a pair of radio means are provided for reversing the phase of the frequency amplifiers are connected in parallel radio frequency output of one of the auxiliary 5 across an antenna, and in which the phase of one tubes. The auxiliary tubes are driven by the 5 of the radio frequency amplifiers is controlled by modulating voltage in such a manner that one amodulatlng voltage. f The 'currentl in the antentube responds to positive peaks of modulating Ana of such a transmitter is varied by the moduvoltage while the other tube responds only to lating voltage so that the resultant antenna curnegative peaks of the modulating voltage. rent is the sum of two radio frequency currents It iS an object of this invention to Provide during positive peaks of modulation, and is equal an improved modulation system. to the difference of the two radio frequency cur- It is a further object 0f this invention to Prorents during negative peaks of the modulating vide a simplified method of adding in phase and voltage. 1 out of phase radio frequency currents in accord- The present practice with respect to transmitance With 8 modulating signal. 15

ter design is to provide a radio frequency power It is a further object of this invention to prooutput stage having sufllcient capacity to handle vide a more economical system of modulating a the power which is developed at 100% modularadio frequency carrier. tion upwards. The average operating output is It is a still further object of this invention to far below the peak value. Consequently, the tube provide an improved system of modulating u re- 20 operating conditions cannot be adjusted to givev dio frequency carrier by means of parallel-conmaximum eincieney during intervals which cornected output tube supplying in phase and out oi respond to the average output. The tubes, as Phase radio frequency currentswell as the associated equipment, must be de- It iS o further object of this invention to prosigned to handle a. peak load which is utilized vide e system for separately generating carrier 25 only a small fraction of the time. This further und Side bend energy. end Subsequently combinincreases' the unit cost of the transmitter. ing them to produce an amplitude modulated In a copending application, Serial No. 210,304, wave. i filed May 27, 1938, Patent No. 2,184,571, Decem- This invention will be better understood when ber 26, 1939, and entitled Modulation system," considered in connection with the accompanying 30 A. W. Vance describes a modulation system which drawing. Its scope is indicated by the appended provides a new approach to the problem 0f 0b. claims. Similar reference numerals are used to taining efficient amplitude modulation. The indicate similar parts throughout the drawing. present invention is an improvement of the sys- Figure 1 iS 9. Schematic drawing of one emwhich is employed in the Vance application and Figure 2 iS c Schematic drawing of an alterna- 35 in the present application, requires the use of a tive embodiment of my invention.

radio 'frequency power output tube which is de- Referring to Fig. 1, unmoduiated radio fresigned toA supply but one-half the maximum load. quency energy iS Supplied t0 a Power output am- 40 An auxiliary tube is called into play to supply an 1Piifiei' 3 frcln a Source Which iS not Shown by 40 irl-phase current which 'increases the eiective means of connections to a pair of input terminals power output during upward peaks ot modula- 5 and 1. The amplifier 3 is a power triode amplition Downward modulation is accompushed by iler designed in the usual manner but rated at a reversing the phase of the radio frequency curmaximum power Output equal Substuntiely to rent supplied by the auxiliary tube. The auxlliene-half the desired transmitter output- Anode 45 ary out of phase current cancels some. or all of Potentiel iS Supplied by u battery 9 which is conthe output from the rst output tube and therenected to the anodey of amplier 3 through a raby reduces the net power which is supplied to the dio frequency choke Il. The output of this amantenna. plifler 3 is connected to the input of an imped- The present invention is an improvement over ance inverter I3. The impedance inverter com- 50 the method described by Vance, in that a simpliprises a pair of capacitors l5 and I1 and an infied system o providing the out of phase radio ductor I9 which are connected to form a quarterfrequency current is shown herein. Instead of wave resonant circuit. The load circuit, which utilizing a balanced modulator and an additional may be an antenna, or the like, is represented by power ampliiier to provide the in and out of a resistor 2l, one side of which is grounded. The 155 output of the impedance inverter is connected to the load resistor through a coupling capacitor 23. As is well known, if a low impedance is connected across the output of an impedance inverter, a high impedance effectively appears across the input. The function of this inversion will be explained subsequently.

Power is also supplied to the load 2| by a pair of auxiilary triodes 25 and'21 which are connected in parallel in the manner which will now be described. The anode 29 of the first auxiliary triode 25 isconnected by means of a coupling capacitor 3| to the load resistor 2|. 'Ihe anode 33 of the second auxiliary triode 21 is connected to the anode 29 through a phase inverter circuit 35. The phase inverter. circuit consists of a pair of inductors 31 and 39 and three capacitors 4|, 43 and 45. One terminal of each inductor is connected to one terminal of capacitor 45.' The remaining terminals of the two inductors are respectively connected to a terminal of the input and output capacitors 4| and 43, respectively. 'I'he remaining terminals of the three capacitors are grounded. The junction of inductor 39 and the input capacitor 4| is connected to the anode 33. The junction of the inductor 31 and the output capacitor 43 is connected to the anode 29.

The inductorsvand capacitors which constitute the phase inverter 35 are so chosen that at the .frequency of operation the phase of a voltage which is applied across the terminals of capacitor 4I .will be opposite to the phase of the voltage which will be induced across the terminals of capacitor 43. This condition is obtained when the circuit constants are chosen so that a half wave resonant filter is obtained, that is the capacitive reactance of the input and output capacitors is equal to the inductive reactance of the two inductors, and the capacitive reactance of the middle capacitor 45 is equal to one half of this value.

Anode potential is supplied by a pair of batteries 41 and 49, or. the like, which are respectively connected to the anodes 29 and 33 through radio frequency chokes 5| and 53.

A push pull audio frequency transformer 55 has its secondary 51 connected between the grids 59 and 5| of the auxiliary triodes. Modulating voltage, from a source which is not shown, is connected to the input terminals 63 and 65 which are connected to the transformer primary.

Radio frequency energy is also supplied to the two auxiliary triodes. In order tol compensate for the 90 degree phase shift which takes place in the impedance inverter I3, it is necessary to provide a similar shift in the phase of the radio frequency current which is applied to the auxiliary triodes. This is accomplished by inserting a phase shifting network 61 in series with a lead which connects the radio frequency input terminal 5 to the center tap of the secondary winding 51. A grid biasing battery 69 is included in series with this lead. The two halves of the secondary winding 51 are bypassed, for radio frequency, by a pair of condensers 1| and 13.

The grid biasing voltage which is applied to the grids of the auxiliary triodes is adjusted to such a value that, in the absence of modulation, substantially no power is delivered to the load by either of the auxiliary triodes. When a modulating voltage is impressed across the terminals of the transformer 55 the voltage is impressed on the grid of each auxiliary triode. Due to the push pull connection. however, when the voltage at any instant on the grid 59 of the rst auxiliary triode is positive, that on the grid 6| of the second auxiliary triode is negative. A positive modulating impulse, therefore, impresses a positive potential on the grid of the first auxiliary triode, permitting it to draw plate current, and to impress a voltage across the load resistor 2| which is in phase with the voltage impressed across the load by fthe radio frequency amplifier 3. A negative modulating impulse, however, causes the second auxiliary triode 21 to draw plate current and the amplified output voltage which is impressed across the load resistor 2| is in the opposite phase due to the action of the phase inverter 35. The result is that there is an increase in the antenna current wheny the modulating pulse is po'sitive, and a decrease in the antenna current when the modulating pulse is negative. Amplitude modulation of the radio frequency carrier is therefore effected in accordance with the modulating voltage.

The impedance inverter I3, which is connected between the output tube 3 and the load, reflects in opposite sense the change in load impedance which is caused by the action of auxiliary triodes. As a result, when the power output is doubled by the in-phase current from the first auxiliary triode, the load impedance which is presented to the output triode 3 changes in such a direction that its output is also effectively doubled. Thus, the condition that the peak output atmaximum modulation be four times the average value is met. f

The embodiment of -my invention which is illustrated in Figure 2 differs from that just described in that the radio frequency voltage is applied out of phase to the twoauxiliary triodes 25 and 21. This makes it unnecessary to invert impedance inverter I3. This provides a direct` connection, for direct current, from the anodes of all the tubes to the radio frequency choke Il, which now supplies anode current for all the amplifiers.

' The modulating voltage is applied out of phase to the two grids 59 and 6| by means of a pushpull transformer 55, as before. The radio frequency input to the auxiliary triodes, however, is impressed on the grids out of phase by'means of a push-pull radio frequency transformer- 15.

The primary 11 of this transformer is connected between the output of the phase shift network 61 and ground. The secondary'19 of the radio frequency transformer 15 is connected between the grids of the auxiliary triodes through a pair of coupling capacitors. and 83. The purpose ofthese capacitors is to prevent the secondary of the radio frequency transformer from shortcircuiting the secondary of the audio frequency transformer 55 which is connected across it. Likewise, to prevent the audio frequency transformer from short-circuiting the radio frequency transformer, a radio frequency choke coil 85 has been connected in series with one side of the secondary winding 51.

in-phase radio frequency voltage to be impressed across the load, while modulating potentials of the other polarity cause out of phase potentials to be impressed across the load."

It was pointed out in fthe Vance application,

1o above' identified, that the plate vonage of the auxiliary tube is varied by the radio frequencyl voltage which is impressed across the load by the unmodulated amplifier. Thus the condition under which in-phase current must be delivered to the antenna is different from lthe condition under which out of phase current must be delivered, for the in-phase current must start to flow when the eective plate voltage is at a minimum and the out of phase current starts l0 when the effective plate vol-tage is at a maximum. In the present system in which two auxiliary tubes are directly coupled to the load, a similar condition exists.

A solution is found, as in the Vance application, by applying unequal bias to the two auxiliary tub, or by supplying a "radio frequency bias. Either of these methods are applicable to my invention, and in addition, I propose another system. By suitably selecting the reactors and capacitors which constitute the phase reversing network a voltage ratio between input and output may be obtained which is greater or less than unity, as desired. An unbalanced condition is therefore providedwhich compen- 35 sates for the difference in plate voltage which appears upon the plates of the tubes.

lI claim:

1. In a device of the character described,a utilization device, a source of unmodulated carrier frequency currents, means for impressing 50 pling the output of the other of said amplifier tubes to said utilization device, whereby modulating voltages of one polarity increase the effective current in said utilization device and of the opposite polarity decrease the effective current in said utilization device.

2. In a device of the character described, a utilization device, a source of unmodulated carrier frequency currents, means for impressing said currents on saidutlization device, a source of modulating voltage, a pair of thermionic amplifier tubes having input and outputA electrodes,

means for applying said modulating voltage to.

said input electrodes in phase opposition, means for applying said unmoduiated carrier frequency currents in phase to said input electrodes, means for directly coupling the output electrode of one of said amplifier tubes to said utilization device, means for coupling the output of the other of said amplifier tubes to said utilization device, said last named means comprising a 180 phase invertning network, whereby modulating voltages of one polarity increase the effective current in said utilization device an'd of the opposite larity decrease the effective current in said ut tion device.

3. A device of the character described in claim 2 in which said 180 phase inverting network has a voltage input to voltage output ratio other than unity.

4. A device of the character described in claim 2 which is further characterized in that said 180 phase shifting network comprises a pair of inductors connected in series between said output electrode and said utilization device having capacitors effectively connected, respectively, between said output electrode and ground, said utilization device and ground, and the junction of said inductors and ground.

5. A device of the character described in claim 2 which is further characterized in that said 180 phase shifting network comprises a pair of similar inductors connected in series between said output electrodes and said utilization device having a capacitor whose capacitive reactance is equal to the inductive reactance of said inductors connected from opposite terminals of said inductors to ground, and a capacitor whose-capacitive reactance is equal to one half the inductive reactance of said inductors connected from the junction of said inductors to ground.

DALE POLLACK. 

